When producing sulfuric acid the acid is in many cases obtained in the form af vapours of H.sub.2 SO.sub.4 in admixture with aqueous vapour (i.e. steam). This is for instance the case in the important sulfuric acid process which consists of a catalytic oxidation, normally over an alkali promoted vanadium catalyst, of SO.sub.2 to form SO.sub.3 which is subsequently hydrated by steam to form sulfuric acid vapour, and that must be condensed in a cooling process. This sulfuric acid process is important, i.a., in connection with the purification of flue gases for sulfur.
When condensing the sulfuric acid vapours, which can take place by cooling under various circumstances, a socalled acid mist (sulfuric acid mist) is formed, i.e. a kind of aerosol consisting of very small droplets of sulfuric acid in the gas mixture from which the sulfuric acid is condensed and which will normally contain steam in a larger amount than that needed for the full hydration of SO.sub.3 into H.sub.2 SO.sub.4. It important to prevent the escape of substantial amounts of the acid mist to the surroundings; for environmental reasons a maximum of escape of sulfuric acid mist is fixed at about 40 mg H.sub.2 SO.sub.4 /Nm.sup.3, corresponding to 9 ppm H.sub.2 SO.sub.4.
It is known to regulate the escape of acid mist by regulating the temperature of the coolant. Thus, there is described in DK patent specification No. 145,457 (U.S. Pat. No. 4,348,373) a process for preparing concentrated sulfuric acid from gases containing SO.sub.3 and excess of H.sub.2 O. The gas is cooled and the sulfuric acid condensed and concentrated in two steps in an absorption tower containing filler bodies. In the lowermost step the feed gas is passed upwardly countercurrently with the condensed acid,the concentration of which is thereby increased In the subsequent step the sulfuric acid vapour is absorbed in sulfuric acid recycled through the layer containing filler bodies. The content of sulfuric acid mist is kept down by virtue of a specified regulation of the temperature; by this the recycle acid is removed from the tower. Remaining acid mist is removed in an aerosol filter placed after the absorption zone. The filter is a "low velocity" filter operating at a linear velocity below 1 m/s and with a pressure drop above 20-30 mbar.
From DK patent application No. 1361/82.(GB patent specification No. 2,117,368) there is known a process for preparing sulfuric acid in which sulfuric acid vapour in gases containing 0.01 to 10% H.sub.2 SO.sub.4 vapour and 0 to 30% steam is condensed in vertical, 9as cooled, acid resistant tubes. The presence of large amounts of acid mist in the gas after the condensing is avoided according to this patent by keeping the temperature difference between the sulfuric acid-containing gas streaming in an upward direction and the coolant streaming in a downward direction (optionally in part crosscurrently) within certain specified limits.
The process known from this publication has been improved according to DK patent application No. 2989/88 so as to be particularly but not exclusively suitable for use when condensing sulfuric acid from gases having a low content of vapours of H.sub.2 SO.sub.4. Such gases may originate from flue gases and other industrial waste gases; SO.sub.2 therein has then been oxidized to SO.sub.3 over a sulfuric acid catalyst and reacted with steam. The condensation takes place in a large number of tubes which each in the top has a small filter causing a drop of pressure of typically 5 to 10 mbar, the gas containing sulfuric acid flowing in an upward direction in countercurrent with the gaseous coolant flowing in a downward direction externally around the tubes. A prerequisite for the ability of this filter to separate the acid droplets/acid mist (and to allow sulfuric acid to flow downwards through the tubes in countercurrent with the gas) down to a residual content of the abovementioned about 40 mg/Nm.sup.3 is that the temperatures TA.sub.1 and TA.sub.2 of the cooling air to and from, respectively, the condenser and the temperature T.sub.2 of gas leaving the tube and filter satisfy the following three conditions: EQU TA.sub.2 &gt;T.sub.d -30-10.alpha..degree.C. (1) EQU T.sub.2 &lt;T.sub.2 *.degree. C. (2) EQU T.sub.2 -TA.sub.1 &lt;90.degree. C. (3)
wherein T.sub.d is the sulfuric acid dew point of the feed gas passed to the tubes, .alpha.is the percentage by volume of H.sub.2 SO.sub.4 in this gas under the assumption that SO.sub.3 is completely hydrated to H.sub.2 SO.sub.4 and T.sub.2 * is the temperature at which the vapour pressure of H.sub.2 SO.sub.4 is about 2 x 10-6 bar in equilibrium with the partial steam pressure prevailing at the outlet of the top of the tubes (corresponding to about 2 ppm sulfuric acid vapour; T.sub.2 * usually is between 100 and 125.degree. C.)
In the following the expression "sulfuric acid tower" means a packed absorption tower as described in U.S. patent specification Ser. No. 4,348,373 (DK 145,457) or a tower containing tubes as described in one of the other publications mentioned above.